The aim of the proposed research is to provide a quantitative description of how reflex movements are organized. The smallest element of this organization is a motor unit. The relative susceptibility of motor units of different types to reflex activation will be studied. Organization of a movement will be described in terms of recruitment order of motor units among the several muscles activated. Muscle response to reflex activation will be studied at three levels of organization: 1. Sequence of activation of motor units of one muscle to graded reflex inputs. 2. Sequence of activation of the motor units of two or more close synergists, responding together to a common reflex input. 3. The effect of mixed simultaneous reflex inputs on the sequence of activation of motoneurons of several muscles. The result will be a description of how motor units of different size and type are utilized in each of several reflex movements.